1. Field of the Invention
The present invention is directed to a method for identifying physiological attempts at respiration, as well as to a respiration detector and a respiratory aid system operating in accordance with the method.
2. Description of the Prior Art
Disease and injuries can make it necessary for a patient to receive respiratory aid. In principle, this respiratory aid can cover everything from facilitating a patient""s spontaneous breathing to complete control of the patient""s breathing. The type and degree of respiratory aid which is supplied depends, on the nature and scope of the disease/injury and the patient""s treatment needs. Patients receiving anaesthesia (regardless of whether it is supplied because of disease/injury, for cosmetic surgery or for some other reason) may also need respiratory aid.
It should be noted that xe2x80x9cpatientxe2x80x9d in this context relates to both people and animals.
Today, the most common way of providing respiratory aid is with ventilators (respirators) delivering a pressurised gas to the lungs. A number of ventilation modes are known, depending on which respiratory aid to supply. See for instance the article xe2x80x9cMechanical Ventilationxe2x80x9d, Chest 1993; 104; 1833-1859, which includes a discussion relating to objectives and recommendations regarding mechanical ventilation.
Another way to provide respiratory aid is to apply extracorporeal pressure around the entire thorax and midriff (e.g. the classic xe2x80x98iron lungxe2x80x99 used for e.g. patients whose respiratory muscles have been damaged by polio). A device of this kind is disclosed in U.S. Pat. No. 5,222,478.
Another way to induce breathing is by means of electrical stimulation. Either by direct stimulation of the muscles of the diaphragm in particular or by indirect stimulation via the nervous system, in particular the phrenic nerve. A diaphragmatic pacer is disclosed in U.S. Pat. No. 5,056,519. A nerve stimulator for treatment of respiratory disorders is disclosed in PCT publication WO 93/01862.
Stimulation of the respiratory system can also be made magnetically, as disclosed in U.S. Pat. No. 5,857,957.
One problem shared by all respiratory aid is to obtain information on when the patient needs to breathe and how much the patient needs to breathe. This is particularly the case for completely controlled respiratory aid with mechanical ventilation.
In completely controlled respiratory aid the physician bears the main responsibility for programming appropriate respiration parameters, such as respiratory rate, tidal volume, inspiration duration etc.
Measurement of physical parameters related to metabolism, such as oxygenation of blood, the carbon dioxide content of blood etc can be of help for the physician when programming the respiration parameters. Some of the physical parameters can be estimated from accurate analysis of the contents of expired gas. The measurements of physiological are too imprecise to provide enough information for individual breath to breath control but can supply important information for the overall regulation of respiratory rate etc.
For patients that can breath spontaneously, at least in part, information of the patients breathing can be determined. When pressure, flow and/or changes in temperature inside or outside the patient""s airways are sensed, the patient""s spontaneous attempts to breathe can be detected and used for triggering the respiratory aid. The magnitude of each attempt to breathe can also be determined and used to affect the magnitude or type of respiratory aid. This option is mainly useful in supported respiration aid.
One problem in supported respiration aid is that the patient""s spontaneous attempts of breathing come in conflict with the supportive respiration aid. This is normally referred to as competition. One way of avoiding competition is to sedate the patient and use completely controlled respiratory aid only.
The above mentioned physical parameters can of course also be used for spontaneously breathing patients for determining when a patient needs to breathe.
Lung movements can also be measured by measuring the impedance across the chest (and lungs). Lung movements are also indicative of attempts to breathe.
Direct measurement of the musculature participating in respiration can also provide information relating to attempts to breathe.
Common to all these procedures is the circumstance that none of them provide an accurate physiological picture of the patient""s true respiratory needs.
Breathing is part of the body""s autonomic system. One important known factor affecting this autonomic system is the body""s own chemical receptors that sense carbon dioxide content. But the autonomic system is also affected by a number of other physiological factors. Some of these factors are probably still unknown to the medical expertise.
An object of the invention is to provide a respiratory aid system designed for determining, more reliably and efficiently than in known systems, when a patient wishes to or needs to breathe.
Another object of the invention is to provide a respiration detector capable of more rapid and efficient sensing of a patient""s attempts at breathing than in known detectors.
Another object of the invention is to provide a method for identifying physiological attempts to breathe.
A patient""s true respiratory need can be established by registering neuroelectrical signals and extracting the signal components related to respiration. In other words, the nerve impulses are detected and used to extract information. The nerve impulses contain information about a patient""s respiratory needs that automatically comprises all physiological functions with an impact on respiration. True respiratory needs can be utilised for controlling a device supplying respiratory aid. In particular, the point in time for each individual breath (initiated by the patient) can then be determined from the neurological signals, but these signals also contain about all the information essential to individual breaths.
In principle, all kinds of known apparatuses for respiratory aid, such as ventilators, anaesthetic machines, nerve stimulators, muscle stimulators and magnetic stimulators, can be used, where or when appropriate with regard to the conditions in each specific case.
Neuroelectrical signals (nerve impulses) related to respiration originate in the respiratory centre of the medulla oblongata, and these signals can, in principle, be collected from the respiratory centre or picked up along the nerves carrying the signals to the respiratory musculature. The phrenic nerve has proved to be particularly suitable for use as a result of its location. Signals can be picked up with an extracorporeal sensor arrangement placed on the skin, with a needle-shaped sensor means adapted to puncture the skin to get close to (or in contact with) the nerve or with a sensor designed to be placed on a surgically exposed nerve. An implanted sensor arrangement could also be considered, in particular for long-term treatment.
One version of the respiratory aid system utilizes sensor signals related to breathing for triggering an inspiration by the patient. The triggering can be adapted so inspiration occurs at a phase in which the patient would have breathed spontaneously. In completely controlled respiratory aid, this would provide a patient-related respiratory rate. In supported respiration, this would also lead to a much faster response to the delivery of respiratory assistance than in prior art technology. Competition between the patient and the ventilator can be avoided.
In cases in which the patient has a varying capacity or ability to breathe spontaneously, a respiratory sensor can be used for determining whether a respiratory signal from the respiratory centre really leads to a spontaneous inspiration or an attempt at inspiration. This kind of respiratory sensor can also be used for determining the strength of an inspiration/attempt at inspiration.
If a spontaneous inspiration does occur, triggering is not always necessary and can be completely inhibited in some instances. The respiratory aid system could instead be devised to provide supplementary support (possibly according to information derived from the neurological signals) for a spontaneous inspiration when the latter is inadequate. For example, the patient""s respiratory musculature might be too weak to sustain physiological inspiration duration.
With a respiration detector for sensing the respiration-related part of the patient""s nervous system, attempts at respiration can be detected at the earliest possible stage and used for controlling a device for supplying respiratory aid.